1. Field of the Invention
The present invention relates to a process for reducing the restart pressure of streams selected from waxy crude oils, water-in-crude emulsions and dispersions of hydrocarbon hydrates, at least partially structured.
2. Description of the Related Art
One of the aspects to be carefully taken into consideration during the development of an oil production plant, is to ensure a continuous and stationary flow of crude oil inside the ducts. Operation stops can occur for several reasons, from simple maintenance to unexpected situations (for example due to a pig block, or a breakdown in a plant for the treatment of crude oil). During the engineering stage, it is therefore necessary to carefully analyse all possible problems which can arise at the restart of production, above all for offshore pipelines (underwater pipes) which, due to their locations, are more difficult to have access to and are characterized by low temperature conditions (0-10° C.).
Structured system means a physical system having a high coordination between its units (molecules or aggregates of soluble or non-soluble molecules, also very extensive) produced by chemical and/or physical bonds. The coordination level depends on the number and strength of the bonds between the structural units. This situation generates an organization that resembles a network including the whole volume occupied by the system (for example a three-dimensional network of regular or amorphous crystals, of a gel, etc.).
The structural level of a substance can be expressed in terms of “yield stress”, defined as the minimum stress (power per surface unit) to be applied to the substance so that this shows a permanent deformation and begins to flow. The “yield stress” is consequently a parameter strongly correlated with the stress (pressure difference at the ends of the pipe) to be applied to allow a stream to pass from a state of stillness to one of motion. The “yield stress” of a substance can be measured experimentally, for example by means of rotational rheometers. Other rheological parameters which allow the structural level of a substance to be quantified are the tensile modulus (G′) and the dissipative modulus (G″). These material parameters are obtained through rheological measurements, effected in a low amplitude oscillatory system. These measurements consist in applying a sinusoidal deformation of variable (and/or constant) frequency and a sufficiently low amplitude as to not disturb the system (“An introduction to Rheology” H. A. Barnes, J. F. Hutton and K. Walters, Elsevier Science Publishers B.V., 1989). The response of the system to the stress will be a sinusoidal signal, out-of-phase with respect to that applied. From measuring the response signal intensity and from the entity of the phase displacement, it is possible to calculate two rheological parameters G′ and G″ which represent the elastic component (solid-type behaviour) and dissipative (liquid behaviour) of the system, respectively. In particular, the tensile modulus represents a parameter which quantitatively expresses (together with the viscosity and the “yield stress”) the structuring degree of the stream considered (“Applied Fluid Rheology”, J. Ferguson, Z. Kemblowski, Elsevier Science Publisher LTD, 1991).
As is well known from literature (A. Uhde, G. Kopp, “Pipeline problems resulting from the handling of waxy crudes”, Journal of the Institute of Petroleum, vol. 57, number 554, 1971; C. Chang, D. V. Boger and Q. D. Nguyen, “Influence of thermal history on the waxy structure of statically cooled waxy crude oil”, SPE Journal 5 (2) June 2000; C. Chang, D. V. Boger “The yielding of waxy crude oils”, Ind. Eng. Chem. Res. 37, 1551-1559. 1998), during the progressive structuring of a fluid put in a state of rest, the “yield stress”, the viscosity and the G′ and G″ modules change proportionally to each other. The two quantities are equivalent material parameters in representing the structuring degree of the stream. With reference to the problems relating to the restart of the stream, however, “yield stress” is the most representative parameter, as it directly expresses the threshold value necessary for generating the flow. Consequently, in the following text, reference will mainly be made to the “yield stress”.
The above-mentioned streams are considered separately hereunder, with the purpose of outlining the specific problems relating to the restart of the duct.
Waxy Crude Oils
The presence of n-paraffins in crude oil can generate wax crystals at temperatures lower than a characteristic temperature of each crude oil, called WAT (Wax Appearance Temperature), which can be defined as the temperature at which the first crystals are observed. The Pour Point (PP), defined as the temperature below which an oil cannot flow under the force of gravity alone, due to its transformation into gel (solid-type behaviour), is found at temperatures lower than the WAT. In operative terms, the PP is measured according to the regulation ASTM D-97 and represents an empirical evaluation of the yield stress.
The WAT of many crude oils, like the PP, is higher than the temperature normally found in deep seabeds (2-3° C.) or in some geographical areas where onshore pipelines are installed.
Under flow-stop conditions, the gelation of crude oil at temperatures lower than the PP, creates a mass of gelled crude oil in a wide tract of the duct, which can generate serious drawbacks during the flow restart operations.
Industry is currently trying to prevent the problem of gelation by:    i) the installation, when possible, of lines and pumps capable of ensuring the necessary pressure in the case of a long and unexpected stoppage of the plant;    ii) the running of the plant, so as to reduce unexpected stoppages;    iii) the use of heated or insulated ducts, so as to reduce the heat exchange;    iv) the use of chemical additives and/or solvents which reduce the tendency or the rate of gelation of waxy crude oils, by improving the properties of the material in terms of viscosity and yield stress.
All these approaches however have various efficacy limitations, mainly in cases of unexpected and prolonged plant-stoppages, or they can be economically unsustainable for the development of the field (for ex. the use of heated pipes), due to the high investment and running costs. Furthermore, the problem can arise in fields already in production, which have been engineered without considering the possibility of the problem arising.
Water-in-Crude Emulsions
The formation of water-in-crude emulsions (defined as emulsions of the W/O type) creates a significant increase in viscosity with respect to the viscosity of the crude oil as such. The increase in viscosity of the water-in-crude emulsion is a function of the volume fraction of the water contained in dispersed form and can be described through relationships such as:ηrelative≡η/ηS=(1+2.5φ+6.2φ2+ . . . )wherein η is the viscosity of the emulsion (W/O), ηS is the viscosity of the continuous phase (oil) alone, ηrelative is called relative viscosity and φ is the volume fraction of the dispersed phase (water).
It is also known that the increase in viscosity, being the same the fraction φ of the dispersed phase, also strongly depends on the particle-size distribution of the latter and on the nature of the interaction between the continuous phase (oil) and the dispersed phase. In the case of W/O emulsions formed with waxy crude oils, there is experimental and field evidence of significant and important increases in the relative viscosity and yield stress with a decrease in the temperature below the WAT of the oil. Under such conditions, serious problems can arise in the emulsion restart.
It is therefore necessary to intervene to reduce the viscosity and yield stress and consequently the restart pressure of these streams.
Dispersions of Hydrocarbon Hydrates
Hydrates are solutions in solid phase of water and other chemical species called guest molecules. The crystalline structure is produced by cages of water molecules (hence the name of clathrate products), in which the guest molecules are kept in a non-stoichiometric manner.
Hydrates can be formed at temperatures significantly higher than the formation temperature of water ice. For example, a 95:5 mixture of water and C1:C3 gaseous hydrocarbons, can form solid hydrates at temperatures slightly below 9° C. and at a pressure of 20 bar. It should be remembered that these operative conditions are not verified solely for particular climatic conditions (deep water and geographical positions with cold climates), but also in the presence of multiphase transport lines with high pressure drops: under these conditions, in fact, hydrocarbon gases generate deep cooling as a result of the Joule-Thomson effect.
The formation of hydrates is a relevant problem, as they can completely obstruct the production lines and, due to the complexity and dangerousness of the removal operations, can cause considerable delays in the production of hydrocarbons and consequently high economical losses.
The most common prevention systems of the formation of hydrates require the use of thermal inhibitors, such as methanol and glycol which, when added to the stream in concentrations equal to about 20% by volume with respect to the water present, lower the formation temperature of hydrates to values outside the operating range. This technique has drawbacks, however, in various production scenarios, among which deep water, due to the anti-economical treatment in the case of high volume fractions of water produced, and the necessity of minimizing plants for the separation and recycling of the thermodynamic inhibitor, mainly in deep water reserves.
Other prevention systems consist of kinetic inhibitors and anti-agglomerating products, prepared to be used at low dosages and disposable. These are chemical products capable of delaying the formation of hydrates or of mitigating their effects by forming hydrate dispersions less compact than the solid which would be formed without the addition of additives and therefore more easily pumpable to the pipeline. Kinetic inhibitors prove to be more advantageous with respect to the conventional techniques (isolated and/or heated lines and the use of methanol or glycol), both in terms of investment and operative costs, but they represent a technology which still has a poorly consolidated efficacy. It should also be noted that, in the presence of hydrate dispersions (formed thanks to the action of anti-agglomerating additives), long flow stoppages (several hours) can lead to significant increases in viscosity and yield stress in the same dispersions, as a result of which serious problems can arise at the re-start. Even in the presence of anti-agglomeration additives, it is therefore important to intervene to reduce the viscosity and re-start pressure.
A stream (whether consisting of waxy crudes, water/crude emulsions or hydrate dispersions, structured or partially structured) maintained under rest conditions and subject to cooling (until a fixed temperature, lower than the starting value, is reached) shows a progressive increase in the viscosity and yield stress which, after a time varying from a few hours to several days, reaches extreme values, characteristic of each stream.